In manufacturing semiconductor devices, there is a plasma etching process of a silicon layer. To be specific, there is an etching process for etching single crystalline silicon to form a groove for burying an oxide film serving as a device isolation film or an etching process for etching polysilicon, i.e. polycrystalline silicon, to form a recess for embedding the oxide film while leaving a gate electrode.
FIGS. 7A to 7C illustrate a process of forming a groove-like space 105, i.e. a recess for burying a device isolation film in a silicon layer 100 of a substrate 101. The silicon layer 100 is etched through a hard mask 102 made of silicon nitride. As shown in FIG. 7A, the hard mask 102 has groove-type openings 103 whose dimension is S and protruded portions 104 between adjacent openings 103 whose dimension is L. Further, the hard mask 102 of the same substrate 101 has a dense pattern region where the dimension S is small and the neighboring protruded portions 104 are densely arranged and a sparse pattern region where the dimension S is large and the neighboring protruded portions 104 are spaced apart from each other.
If the substrate 101 is etched by using a processing gas, e.g., Cl2 gas, by-products containing, e.g., silicon and chlorine are generated. Some of the by-products are vaporized and discharged from the openings 103, but the rest of them are deposited as deposits 106 on the sidewall indicated as lines 107 between the spaces 105, as shown in FIG. 7B. Since the dimension of the lines 107 is getting larger due to this, the dimension S′ of the spaces 105 after the etching process becomes smaller than the dimension S of the openings 103 formed through the hard mask 102, as shown in FIG. 7C.
Meanwhile, if the hard mask 102 has the dense and sparse pattern regions of the protruded portions 104 as described above, the volume of one single recess (opening) 103 in the sparse pattern region is larger than that in the dense pattern region. Therefore, more by-products are generated in a single recess of the sparse pattern region than in a single recess of the dense pattern region. Further, since the amount of the deposits 106 increase as the amount of the by-products increase, the amounts of the deposits 106 in the sparse pattern region are larger than those in the dense region.
As a result, the sidewall tilt angle β of the lines 107 in the sparse pattern region is smaller than the sidewall tilt angle α of the lines 107 in the dense pattern region, as shown in FIG. 7C. As the difference of the dimensions S of the openings 103 in the dense and sparse pattern regions increases, the amount of the difference of the generated by-products becomes larger, which results in the increase of the difference between the sidewall tilt angles α and β of the lines 107. Furthermore, since the deposits 106 are more easily deposited in the sparse pattern region than in the dense pattern region, if the amounts of the by-products increase, the difference between the sidewall tilt angles α and β of the lines 107 in the dense and sparse pattern regions increases, as shown in FIG. 8.
If the sidewall tilt angles α and β of the lines 107 after the etching process in the dense and sparse pattern regions are unbalanced as mentioned above, and then for example, if the hard mask 102 is removed and an oxide film is embedded in the space 105, burying properties (filling rate) of the oxide film in the space 105 of the oxide film cannot be uniform. Moreover, if the filling rate of the oxide film is not uniform, electrical characteristics such as oxide film withstanding voltage characteristics may also not be uniform in surface. Therefore, it is required to make the sidewall tilt angles α and β of the lines 107 in the dense and sparse pattern regions equal to each other.
Conventionally, the etching process is performed under the processing conditions controlled such that the deposits 106 are prevented to be deposited on the sidewall of the lines 107 or the deposits 106 are to be removed. That is, the processing conditions are adjusted so that the sidewall tilt angles α and β of the lines 107 are very large (close to 90°) and thereby the difference between the sidewall tilt angles α and β of the lines 107 in the dense and sparse pattern regions becomes small.
However, since it is preferable that the sidewall tilt angles α and β of the lines 107 are small, i.e. reclined, for the enhancement of the burying properties of the oxide film described above, it is required to make the sidewall tilt angles α and β in the dense and sparse pattern regions equal to each other and at the same time to make the tilt angles α and β small. Furthermore, techniques for freely controlling the sidewall tilt angles α and β of the lines 107 are also required.
Japanese Patent Laid-open Application No. 1996-115900 (paragraphs [0021] and [0022]) discloses a technique for etching a silicon-based layer into a groove shape by using Cl2 gas and CO gas, but the difference in shape of grooves or the tilt angles of the lines 107 in the dense and sparse pattern regions are not considered.